The Doppler effect is widely used in monitoring systems to measure the speed of moving objects. Some examples include measurement of blood flow with medical ultrasound equipment and determination of aircraft movement with radar. The Doppler effect causes the frequency of a wave reflected from a moving object to shift relative to the frequency of the wave directed at the object. The amount of frequency shift is determined by the speed of the object.
FIG. 1 illustrates a basic system for measuring Doppler frequency shift. A master oscillator 10 generates a reference frequency signal that is amplified by transmit (Tx) amplifier 12 to generate an electrical signal that drives a transmit element 14. The transmit element emits waves that are reflected by an object of interest 16 and return to a receive (Rx) element 18 where they are converted to electrical signals. A receive amplifier 20 boosts the power of the Rx signal which is then demodulated by a mixer 22 that mixes the Rx signal with a reference signal derived from the master oscillator. A filter stage 24 having high-pass and low-pass filters (HPF and LPF) removes unwanted components from the demodulated signal before further processing, typically by a digital signal processing system.
The transmit and receive elements are often mounted together in a single transducer. In a radar system, the transmit element may be embodied as an antenna which converts the electrical signals into electromagnetic waves. The receive element then converts the reflected electromagnetic waves back into electrical signals. In an ultrasound system, the transmit and receive elements may be realized as crystals which convert electrical signals into sound waves and vice versa.
The system of FIG. 1 is known as a coherent demodulation system because the reference signal for the demodulator is derived from the same master oscillator used to generate the transmit signal. It is a non-directional system because it cannot distinguish between motion towards the transducer and motion away from the transducer. It can only determine the magnitude (speed) of the motion. This is because motion of the object towards and away from the transducer create upper and lower Doppler sidebands in the carrier frequency spectrum which are then shifted into the same region of the baseband output from the demodulator. The Rx signal may be expressed as follows:
                              S          ⁡                      (            t            )                          =                                                            A                0                            ⁢                              cos                ⁡                                  (                                                                                    ω                        0                                            ⁢                      t                                        +                                          φ                      0                                                        )                                                                    ︸              Carrier                                +                                                    A                f                            ⁢                              cos                ⁡                                  (                                                                                    ω                        0                                            ⁢                      t                                        +                                                                  ω                        f                                            ⁢                                                                                          ⁢                      t                                        +                                          φ                      f                                                        )                                                                    ︸              Forward                                +                                                    A                r                            ⁢                              cos                ⁡                                  (                                                                                    ω                        0                                            ⁢                      t                                        -                                                                  ω                        r                                            ⁢                      t                                        +                                          φ                      r                                                        )                                                                    ︸              Reverse                                                          Eq        .                                  ⁢        1            where A, ω, and φ represent the amplitude, angular frequency and phase of each signal component, and the subscripts 0, f, and r signify the carrier, forward (toward the transducer probe), and reverse (away from the probe) components. The system of FIG. 1 is unable to distinguish between the forward and reverse sidebands.
To determine the direction in which the object is moving (i.e., towards or away from the transceiver), a more sophisticated demodulation technique is required. Some examples of directional demodulation techniques include single side-band demodulation (SSB), heterodyne demodulation, and quadrature demodulation.
FIG. 2 illustrates a basic Doppler measurement system that utilizes quadrature demodulation to obtain directional information on the movement of an object. The system of FIG. 2 is similar to that of FIG. 1, but the demodulator includes two mixers 22 and 26 which mix the Rx signal with an in-phase (“I”) clock signal from the master oscillator, and a quadrature (“Q”) clock signal that is phase shifted (90 degrees) from the I clock signal. Thus, a quadrature demodulator is also referred to as an I/Q demodulator.
After high-pass and low-pass filtering in filter stages 24 and 28, the demodulated signals I′(t) and Q′(t) contain only the Doppler components:
                                          I            ′                    ⁡                      (            t            )                          =                                            1              2                        ⁢                          A              f                        ⁢                          cos              ⁡                              (                                                                            ω                      f                                        ⁢                    t                                    +                                      ϕ                    f                                                  )                                              +                                    1              2                        ⁢                          A              r                        ⁢                          cos              ⁡                              (                                                                            ω                      r                                        ⁢                    t                                    +                                      ϕ                    r                                                  )                                                                        Eq        .                                  ⁢        2                                                      Q            ′                    ⁡                      (            t            )                          =                                            -                              1                2                                      ⁢                          A              f                        ⁢                          sin              ⁡                              (                                                                            ω                      f                                        ⁢                    t                                    +                                      ϕ                    f                                                  )                                              +                                    1              2                        ⁢                          A              r                        ⁢                          sin              ⁡                              (                                                                            ω                      r                                        ⁢                    t                                    +                                      ϕ                    r                                                  )                                                                        Eq        .                                  ⁢        3            Further processing extracts the forward and reverse components from the I′(t) and Q′(t) signals.
Transducers having multiple transmit and/or receive elements may be used to improve the basic performance of a Doppler measurement system, or to provide additional functionality. For example, multiple element transmitters and receivers may be used to implement beamforming techniques in which the maximum transmit and receive strength of the transducer are pointed in the direction of the object to be measured. The elements are arranged in an array that is scanned by a beamforming network so that the signal to or from each element is phase (or time) shifted relative to the other elements. In the transmit path, a beamforming network shifts the transmit signal by different phase amounts as it distributes the signal to the different elements in the scanning process. In the receive path, a beamforming network shifts the receive signals by different amounts as the array is scanned so the signals from the individual elements can be phase (or time) aligned and then combined (summed) to form a single receive signal. The summed signal level theoretically thereby increases by N where N is the number of array elements. If noise is uncorrelated, it increases by √{square root over (N)}, thereby increasing the summed signal-to-noise ratio (SNR) by N/√{square root over (N)}=√{square root over (N)}.
FIG. 3 illustrates the basic principle of using an array of elements for beamforming. In this example, which is generic to both transmit and receive scanning, the signals for array elements A1 through AN are shown shifted by various amounts relative to time t=0. The amount each signal is shifted during the scanning process determines the angle Θ of the beam. Numerous scanning techniques have been developed including time-delay scanning and phase scanning; if done in the analog domain, it is called analog beamforming, if done in the digital domain it is called digital beamforming (DBF). There are two general ways to beamform. One is to align and sum the signals at the RF frequency as shown in FIG. 4. The other is at baseband, in which case each channel requires downconversion and filtering before summation.
FIG. 4 illustrates a beamforming system for time-delay scanning an array of receive elements in an ultrasound system that utilizes RF summation followed by quadrature demodulation. The receiver array 30 includes multiple receive crystals A1 through AN (where N is typically a power of 2) which convert the received ultrasound waves into electrical signals that are amplified by low noise amplifiers (LNAs) 34. The outputs from the LNAs are converted to current-mode signals by V-I converters 36. A multiplexer array 38 selectively connects the current-mode signals to the input taps of a delay line circuit 40. The delay line circuit imparts a different time delay to each input signal depending on which input tap it is applied to. All of the delayed input signals are then summed at node N1 to produce a composite receive signal which is then quadrature demodulated by multipliers 42 and 44 in response to the I clock signal cos(ω0t) and the Q clock signal sin(ω0t). The resulting signals are then band-pass filtered and converted to digital form for further processing.
Although the system of FIG. 4 may provide effective beamforming and demodulation, it generally requires a very high performance demodulator which drives up the system power and cost. Also, since delay line circuits are generally cumbersome to add to each individual receive element, the system includes multiplexer array 38 to allow the use of a single delay line circuit where the currents are routed to the appropriate tap to insure coherent addition at node N1. In a DBF system, a separate memory for each channel functions as the “delay line”. Selecting the appropriate memory locations allows multiple beams to be formed at the same time. However, continuous wave (CW) Doppler signals tend to have dynamic ranges that are too large to be processed by a DBF system. In addition, multiplexer arrays tend to require extensive space on an integrated circuit.
FIG. 5 illustrates a beamforming system for phase scanning an array of receive elements in an ultrasound system that utilizes quadrature demodulation. The system of FIG. 5 includes an array of receive crystals A1 through AN and low noise amplifiers (LNAs) 34. The output from each LNA is demodulated by an I mixer 46 and a Q mixer 48 in response to I and Q clock signals (which are not shown here for simplicity). Phase rotators 50 and 52 are placed in series after the mixers to phase shift the I1 . . . N and Q1 . . . N signals in response to the phase select signals Φ1 through ΦN. The I1 . . . N and Q1 . . . N outputs are summed to generate I(t) and Q(t) which may then be converted to voltage mode signals, band-pass filtered, etc.
Although the system of FIG. 5 requires a complete demodulator for each receiver, each of the demodulators is generally subject to less demanding criteria than the demodulator required for the system in FIG. 4 since they are located before beamformation. Another salient feature is that the phase rotation is performed at baseband which is generally less demanding than operations performed at the full RF frequencies of the input signals. The system in FIG. 5 is expected to require less circuit board space than the system of FIG. 4.